Touch panels have become widely adopted as the input device for a range of electronic products such as smart-phones and tablet devices.
Most high-end portable and handheld electronic devices now include touch panels. These are most often used as part of a touchscreen, i.e., a display and a touch panel that are aligned so that the touch zones of the touch panel correspond with display zones of the display.
The most common user interface for electronic devices with touchscreens is an image on the display, the image having points that appear interactive. More particularly, the device may display a picture of a button, and the user can then interact with the device by touching, pressing or swiping the button with their finger or with a stylus. For example, the user can “press” the button and the touch panel detects the touch (or touches). In response to the detected touch or touches, the electronic device carries out some appropriate function. For example, the electronic device might turn itself off, execute an application, or the like.
Although a number of different technologies can be used to create touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
A well-known approach to capacitive sensing applied to touch panels is the projected capacitive approach. This approach includes the mutual-capacitance method and the self-capacitance method.
In the mutual-capacitance method, as shown in FIG. 1, a drive electrode 100 and sense electrode 101 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 100 from a voltage source 102. A signal is then generated on the adjacent sense electrode 101 by means of capacitive coupling via the mutual coupling capacitor 103 formed between the drive electrode 100 and sense electrode 101. A current measurement unit or means 104 is connected to the sense electrode 101 and provides a measurement of the size of the mutual coupling capacitor 103. When the input object 105 (such as a finger or stylus) is brought into close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 106 and a second dynamic capacitor to the sense electrode 107. If the input object is connected to ground, as is the case for example for a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement unit or means 104 attached to the sense electrode 101.
In the self-capacitance method, as shown in FIG. 2, a drive electrode 200 is formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 200 from a voltage source 201. A current measurement means 202 is connected to the electrode 200 and provides a measurement of the size of the self-capacitance 203 of the electrode to ground. When the input object 105 is brought into close proximity to the electrode, it changes the value of the self-capacitance 203. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect is to increase the self-capacitance of the electrode to ground 203 and hence to increase the magnitude of the signal measured by the current measurement means 202 attached to the sense electrode 200.
As is well-known and disclosed, for example, in U.S. Pat. No. 5,841,078 (Bisset et al, issued Oct. 30, 1996), by arranging a plurality of drive and sense electrodes in a grid pattern to form an electrode array, the mutual-capacitance sensing method may be used to form a touch panel device. FIG. 3 shows a suitable pattern of horizontal electrodes 300 that may be configured as drive electrodes, and vertical electrodes 301 that may be configured as sense electrodes. An advantage of the mutual-capacitance sensing method is that multiple simultaneous touch input events may be detected.
It is well-known that by arranging a plurality of electrodes in a grid pattern to form an electrode array, the self-capacitance sensing method may be used to form a touch panel device. FIG. 3 shows a suitable pattern of horizontal electrodes 300 and vertical electrodes 301 that may be configured as sense electrodes. However, a limitation of such a device is that it cannot reliably detect simultaneous touches from multiple objects.
It is also well-known and disclosed, for example, in U.S. Pat. No. 9,250,735 (Kim et al, issued Feb. 2, 2016), that by arranging a plurality of electrodes in a two dimensional array, and by providing an electrical connection from each electrode to a controller, this self-capacitance sensing method may be used to form a touch panel device that is able to reliably detect simultaneous touches from multiple objects. Mutual capacitance sensing may also be used with such a two dimensional array of separately-connected electrodes, for example as disclosed in US 2016/0320886 (Kim et al, published Nov. 3, 2016).
In many touch screens the touch panel is a device independent of the display, known as an “out-cell” touch panel. The touch panel is positioned on top of the display, and the light generated by the display crosses the touch panel, with an amount of light being absorbed by the touch panel. In more recent implementations, part of the touch panel is integrated within the display stack, and touch panel and display may share the use of certain structures, such as transparent electrodes. This is known as an “in-cell” touch panel. This integration of the touch panel into the display structure seeks to reduce cost by simplifying manufacture, as well as reducing the loss of light throughput that occurs when the touch panel is independent of the display and located on top of the display stack.
A limitation of the capacitance measurement techniques described above as conventionally applied to touch panels is that they are incapable of detecting input from non-conductive or insulating objects, for example made of wood, plastic or the like. A non-conductive object that has a dielectric permittivity different to air will cause the measured array capacitances to change when in close proximity to the touch panel surface. However, the magnitude of the resulting signal is very small—for example, less than 1% of that generated by a conductive object—and is dependent on the type of material the non-conductive object is made of and the ambient environment conditions. This disadvantageously reduces the usability of the touch panel since it is restricted to operation using conductive input objects, such as a finger or metallic pen or stylus. In particular, the user cannot operate a touch panel reliably while wearing normal (non-conductive) gloves or while holding a non-conductive object such as a plastic pen.
U.S. Pat. No. 9,105,255 (Brown et al, issued Aug. 11, 2015) discloses a type of mutual-capacitance touch panel that is able to detect non-conductive objects, and to distinguish whether an object is conductive or non-conductive. This is achieved by measuring multiple mutual capacitances formed over different coupling distances. The type of object (conductive or non-conductive) can be determined based on the changes in the multiple mutual capacitances. The multiple mutual capacitances are formed between an array of row and column electrodes.
Commonly owned U.S. patent application Ser. No. 15/409,910 discloses a method for detecting non-conductive objects, or for distinguishing between conductive and non-conductive objects, using a two dimensional array of electrodes, each of which have a separate connection to a controller. The controller measures the mutual capacitance between groups of electrodes during multiple measurement periods. In each measurement period, the controller assigns some electrodes as drive electrodes, some electrodes as sense electrodes, and some electrodes as unused electrodes. The controller applies a drive signal to the drive electrodes, and measures the coupling between the drive electrodes and each sense electrode.